Woodlouse vs Pill Bug
Armadillidium vulgare vs Oniscus asellus
If it curls into a complete ball, it is a pill bug. If it flattens and runs, it is a sow bug. Both are crustaceans, both are entirely harmless, and one of them is considerably better in a terrarium.
The one thing that settles it
Does it roll into a sphere. A pill bug closes into a complete, sealed ball with nothing showing. A sow bug cannot — it flattens against the surface and runs instead. There is no ambiguity and no equipment needed, which makes this one of the cleanest identifications on the site.
Side by side
| Character | Pill Bug | Common Woodlouse |
|---|---|---|
| Danger | Harmless | Harmless |
| When disturbed | Rolls into a sealed sphere | Flattens and runs |
| Rear end | Rounded, with no projections | Two small pointed tail projections |
| Body shape | Strongly domed, almost hemispherical | Flatter and more oval |
| Tolerance of dry air | Better — the ball reduces water loss | Poorer, so it stays closer to constant damp |
| In a terrarium | The standard beginner isopod | Kept less often, and escapes more readily |
| Danger | None whatsoever | None whatsoever |
Why they get confused
They are close relatives with the same body plan, they live in the same places, and both get called woodlice, sow bugs, rollie pollies and pill bugs more or less interchangeably depending on where you grew up. The names are the problem more than the animals are — pill bug and sow bug are the two useful terms, and woodlouse is the umbrella covering both.
If you still can’t tell
Touch it gently with a fingertip and wait. That is the whole test, it takes two seconds, and it does not hurt the animal. If it is already rolled up when you find it, it is a pill bug — sow bugs cannot do it at all. Either way there is nothing to worry about: neither bites, stings, carries anything or damages a building, and both are useful in a compost heap.
Common questions
Are they insects?
No, and this surprises most people. Both are crustaceans, more closely related to crabs and shrimp than to anything with six legs, and both breathe through gill-like structures.
Why are they in my house?
They came in looking for damp and are dying. Neither can breed indoors in a heated house. Repeated appearances point at moisture getting in somewhere, which is the thing worth fixing.
Do they damage plants?
Rarely. Both prefer decaying material, and damage to living plants usually means there is nothing dead left for them to eat.
Full records
- Pill Bug Harmless
- Common Woodlouse Harmless
Where this was checked
- iNaturalist — Armadillidium vulgare checked 2026-07-30
- iNaturalist — Oniscus asellus checked 2026-07-30
Last reviewed 2026-07-30. We identify, we don’t diagnose — what we can and can’t tell you.